Recent experimental evidence suggests that the outer hair cells of the mammalian cochlea act as electromechanical amplifiers which increase hearing sensitivity one-hundred fold. The long term goal of the proposed research is to confirm this hypothesis and to clarify our understanding of the underlying mechanisms. The specific aim of this proposal is to obtain experimental evidence in support of a detailed hypothesis describing the function of the cochlear amplifier. Our specific hypothesis is that the outer hair cell receptor current regulates a force-generation process which is located in the hair cell soma. This force generation process is coupled to two modes of traveling-wave propagation. The velocities of the two modes differ except over a limited cochlear region where they are equal. In this region of equal velocity amplification takes place and the mechanical stimulus to the inner hair cells is significantly increased. We plan to characterize the forces produced by outer hair cells by stimulating voltage-dependent outer hair cell length changes through the injection of alternating-current into the scala media of the cochlea. The resulting mechanical response will be measured both as an otoacoustic emission as well as movement of the basilar membrane with a fiber-optic displacement probe. We will also study the effects of other cochlear manipulations such as efferent stimulation and simultaneous acoustic stimulation. We will search for direct evidence of a second mode of traveling wave propagation by measuring stiffness with our force probe and displacement gradients with our displacement probe. The results of the proposed experiments as well as previous experiments by ourselves and others will be interpreted with the aid of computational models. Two types of models will be used: micromechanical and macromechanical. The computational models will be physically based and include specific descriptions of the mechanical properties of the organ of Corti and tectorial membrane as well as hair cell mechano-electric transduction and outer hair cell motility.